TRS-80 Model 4p

Ports

Address     Read                    Write
FC-FF       Cassette in             Cassette out, resets
F8-FB       Rd printer status       Wr to printer
F4-F7       -                       Drive select
F3          FDC data reg            FDC data reg
F2          FDC sector reg          FDC sector reg
F1          FDC track reg           FDC track reg
F0          FDC status reg          FDC cmd reg
EC-EF       Reset RTC INT           Mode output
EB          RS232 recv holding reg  RS232 xmit holding reg
EA          UART status reg         UART/modem control
E9          -                       Baud rate register
E8          Modem status            Master reset/enable
                                    UART control reg
E4-E7       Rd NMI status           Wr NMI mask reg
E0-E3       Rd INT status           Wr INT mask reg
CF          HD status               HD cmd
CE          HD size/drv/hd          HD size/drv/hd
CD          HD cylinder high        HD cylinder high
CC          HD cylinder low         HD cylinder low
CB          HD sector #             HD sector #
CA          HD sector cnt           HD sector cnt
C9          HD error reg            HD write precomp
C8          HD data reg             HD data reg
C7          HD CTC chan 3           HD CTC chan 3
C6          HD CTC chan 2           HD CTC chan 2
C5          HD CTC chan 1           HD CTC chan 1
C4          HD CTC chan 0           HD CTC chan 0
C2-C3       HD device ID            -
C1          HD control reg          HD Control reg
C0          HD wr prot reg          -
94-9F       -                       -
90-93       -                       Sound option
8C-8F       Graphic sel 2           Graphic sel 2
8B          CRTC Data reg           CRTC Data reg
8A          CRTC Control reg        CRTC Control reg
89          CRTC Data reg           CRTC Data reg
88          CRTC Control reg        CRTC Control reg
84-87       -                       Options reg
83          -                       Graphic X reg
82          -                       Graphic Y reg
81          Graphics RAM            Graphics RAM
80          -                       Graphics options reg

Bit map

Address     D7      D6      D5      D4      D3      D2      D1      D0
F8-FB-Rd    Busy    Paper   Select  Fault   -       -       -       -
EC-EF-Rd    (any read causes reset of RTC interrupt)
EC-EF-Wr    -       CPU     -       Enable  Enable  Mode    Cass    -
                    Fast            EX I/O  Altset  Select  Mot on
E0-E3-Rd    -       Recv    Recv    Xmit    10 Bus  RTC     C Fall  C Rise
                    Error   Data    Empty   int     Int     Int     Int
E0-E3-Wr    -       Enable  Enable  En.Xmit Enable  Enable  Enable  Enable
                    Rec err Rec dat Emp     10 int  RTC int CF int  CR int
90-93-Wr    -       -       -       -       -       -       -       Sound
                                                                    Bit
84-87-Wr    Page    Fix upr Memory  Memory  Invert  80/64   Select  Select
                    mem     bit 1   bit 0   video           Bit 1   Bit 0

System memory map

Memory map 1 - model III mode

0000-1fff       ROM A (8K)
2000-2fff       ROM B (4K)
3000-37ff       ROM C (2K) - less 37e8/37e9
37e8-37e9       Printer Status Port
3800-3bff       Keyboard
3c00-3fff       Video RAM (page bit selects 1K or 2K)
4000-7fff       RAM (16K system)
4000-ffff       RAM (64K system)

Memory map 2

0000-37ff       RAM (14K)
3800-3bff       Keyboard
3c00-3fff       Video RAM
4000-7fff       RAM (16K) end of one 32K bank
8000-ffff       RAM (32K) second 32K bank

Memory map 3

0000-7fff       RAM (32K) bank 1
8000-f3ff       RAM (29K) bank 2
f400-f7ff       Keyboard
f800-ffff       Video RAM

Memory map 4

0000-7fff       RAM (32K) bank 1
8000-ffff       RAM (32K) bank 2

TRSDOS memory map

0000-25ff       Reserved for TRSDOS operations
2600-2fff       Overlay area
3000-HIGH       Free to use
HIGH-ffff       Drivers, filters, etc

Use `MEMORY` command to know value of `HIGH`

Supervisor calls

SVC are made by loading the correct SVC number in A, other params in other regs, and then call rst 0x28.

Z is pretty much always used for success or as a boolean indicator. It is sometimes not specified when there's not enough tabular space, but it's there. When - is specified, it means that the routine either never returns or is always successful.

Num Name    Args                Res  Desc
00  IPL     -                   -    Reboot the system
01  KEY     -                   AZ   Scan *KI, wait for char
02  DSP     C=char              AZ   Display character
03  GET     DE=F/DCB            AZ   Get one byte from device or file
04  PUT     DE=F/DCB C=char     AZ   Write one byte to device or file
05  CTL     DE=DBC C=func       CAZ  Output a control byte
06  PRT     C=char              AZ   Send character to printer
07  WHERE   -                   HL   Locate origin of SVC
08  KBD     -                   AZ   Scan keyboard and return
09  KEYIN   HL=buf b=len c=0    HLBZ Accept a line of input
0a  DSPLY   HL=str              AZ   Display message line
0b  LOGER   HL=str              AZ   Issue log message
0c  LOGOT   HL=str              AZ   Display and log message
0d  MSG     DE=F/DCB HL=str     AZ   Send message to device
0e  PRINT   HL=str              AZ   Print message line
0f  VDCTL   special             spc  Video functions
10  PAUSE   BC=delay            -    Suspend program execution
11  PARAM   DE=ptbl HL=str      Z    Parse parameter string
12  DATE    HL=recvbuf          HLDE Get date
13  TIME    HL=recvbuf          HLDE Get time
14  CHNIO   IX=DCB B=dir C=char -    Pass control to next module in device chain
15  ABORT   -                   -    Abort Program
16  EXIT    HL=retcode          -    Exit to TRSDOS
18  CMNDI   HL=cmd              -    Exec Cmd w/ return to system
19  CMNDR   HL=cmd              HL   Exec Cmd
1a  ERROR   C=errno             -    Entry to post an error message
1b  DEBUG   -                   -    Enter DEBUG
1c  CKTSK   C=slot              Z    Check if task slot in use
1d  ADTSK   C=slot              -    Remove interrupt level task
1e  RMTSK   DE=TCB C=slot       -    Add an interrupt level task
1f  RPTSK   -                   -    Replace task vector
20  KLTSK   -                   -    Remove currently executing task
21  CKDRV   C=drvno             Z    Check drive
22  DODIR   C=drvno b=func      ZBHL Do directory display/buffer
23  RAMDIR  HL=buf B=dno C=func AZ   Get directory record or free space
28  DCSTAT  C=drvno             Z    Test if drive assigned in DCT
29  SLCT    C=drvno             AZ   Select a new drive
2a  DCINIT  C=drvno             AZ   Initialize the FDC
2b  DCRES   C=drvno             AZ   Reset the FDC
2c  RSTOR   C=drvno             AZ   Issue a FDC RESTORE command
2d  STEPI   C=drvno             AZ   Issue a FDC STEP IN command
2e  SEEK    C=drvno DE=addr     -    Seek a cylinder
2f  RSLCT   C=drvno             -    Test for drive busy
30  RDHDR   HL=buf DCE=addr     AZ   Read a sector header
31  RDSEC   HL=buf DCE=addr     AZ   Read a sector 
32  VRSEC   DCE=addr            AZ   Verify sector
33  RDTRK   HL=buf DCE=addr     AZ   Read a track 
34  HDFMT   C=drvno             AZ   Hard disk format
35  WRSEC   HL=buf DCE=addr     AZ   Write a sector 
36  WRSSC   HL=buf DCE=addr     AZ   Write system sector 
37  WRTRK   HL=buf DCE=addr     AZ   Write a track 
38  RENAM   DE=FCB HL=str       AZ   Rename file
39  REMOV   DE=D/FCB            AZ   Remove file or device
3a  INIT    HL=buf DE=FCB B=LRL AZ   Open or initialize file
3b  OPEN    HL=buf DE=FCB B=LRL AZ   Open existing file or device
3c  CLOSE   DE=FCB/DCB          AZ   Close a file or device
3d  BKSP    DE=FCB              AZ   Backspace one logical record
3e  CKEOF   DE=FCB              AZ   Check for EOF
3f  LOC     DE=FCB              BCAZ Calculate current logical record number
40  LOF     DE=FCB              BCAZ Calculate the EOF logical record number
41  PEOF    DE=FCB              AZ   Position to end of file
42  POSN    DE=FCB BC=LRN       AZ   Position file
43  READ    DE=FCB HL=ptr       AZ   Read a record
44  REW     DE=FCB              AZ   Rewind file to beginning
45  RREAD   DE=FCB              AZ   Reread sector
46  RWRIT   DE=FCB              AZ   Rewrite sector
47  SEEKSC  DE=FCB              -    Seek cylinder and sector of record
48  SKIP    DE=FCB              AZ   Skip a record
49  VER     DE=FCB              HLAZ Write and verify a record
4a  WEOF    DE=FCB              AZ   Write end of file
4b  WRITE   DE=FCB HL=ptr       AZ   Write a record
4c  LOAD    DE=FCB              HLAZ Load program file
4d  RUN     DE=FCB              HLAZ Run program file
4e  FSPEC   HL=buf DE=F/DCB     HLDE Assign file or device specification
4f  FEXT    DE=FCB HL=str       -    Set up default file extension
50  FNAME   DE=buf B=DEC C=drv  AZHL Get filename
51  GTDCT   C=drvno             IY   Get drive code table address
52  GTDCB   DE=devname          HLAZ Get device control block address
53  GTMOD   DE=modname          HLDE Get memory module address
55  RDSSC   HL=buf DCE=addr     AZ   Read system sector 
57  DIRRD   B=dirent C=drvno    HLAZ Directory record read
58  DIRWR   B=dirent C=drvno    HLAZ Directory record write
5a  MUL8    C*E                 A    Multiply C by E
5b  MUL16   HL*C                HLA  Multiply HL by C
5d  DIV8    E/C                 AE   Divides E by C
5e  DIV16   HL/C                HLA  Divides HL by C
60  DECHEX  HL=str              BCHL Convert Decimal ASCII to binary
61  HEXDEC  HL=num DE=buf       DE   Convert binary to decimal ASCII
62  HEX8    C=num HL=buf        HL   Convert 1 byte to hex ASCII
53  HEX16   DE=num HL=buf       HL   Convert 2 bytes to hex ASCII
64  HIGH$   B=H/L HL=get/set    HLAZ Get or Set HIGH$/LOW$
65  FLAGS   -                   IY   Point IY to system flag table
66  BANK    B=func C=bank       BZ   Memory bank use
67  BREAK   HL=vector           HL   Set Break vector
68  SOUND   B=func              -    Sound generation

Personal reverse engineering

This section below contains notes about my personal reverse engineering efforts. I'm not an expert in this, and also, I might not be aware of existing, better documentation making this information useless.

Bootable disk

I'm wondering what makes a disk bootable to the TRS-80 and how it boots it. When I read the raw contents of the first sector of the first cylinder of the TRS-DOS disk, I see that, except for the 3 first bytes (00fe14), the rest of the contents is exactly the same as what is at memory offset 0x0203, which seems to indicates that the bootloader simply loads that contents to memory, leaving the first 3 bytes of RAM to either random contents or some predefined value (I have f8f800).

A non-bootable disk starts with 00fe14, but we can see the message "Cannot boot, DA TA DISK!" at offset 0x2a.

I'm not sure what 00fe14 can mean. Disassembled, it's nop \ rst 0x28 \ ld b, c. It makes sense that booting would start with a service call with parameters set by the bootloader (so we don't know what that service call actually is), but I'm not sure it's what happens.

I don't see any reference to the 0x2a offset in the data from the first sector, but anyways, booting with the non-bootable disk doesn't actually prints the aformentioned message, so it might be a wild goose chase.

In any case, making a disk bootable isn't a concern as long as Collapse OS uses the TRS-DOS drivers.

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